The bottom (vertical rack height adjustment) slider assembly is simple. Just slide on the clamps, insert the shorter tubes and just snug up the clamp screws enough to keep the sliding tubes from falling out.
Assemble the upper (horizontal or front-to-rear rack position adjustment) clamps on the rack cross tube per picture. Each clamp consists of a barrel, an upper tube block, a lower tube block and a set screw. The pair of tube blocks align with the direction of the slider tube or the rack cross tube. The concave surfaces face the tubes and prevent crushing and deforming the tubing under clamp pressure when the set screws are tightened.
After the rack is temporarily assembled, bolt it to your frame's existing hard points loosely. Adjust the four sliding tubes to level the rack. This will require simultaneous movement of all four sliding tubes. Tighten all four clamps bolts on the lower (vertical) slider clamps and the two set screws on the barrel clamps (horizontal sliders). Tighten the screws/bolts on the frame's dropout eyelets and the tabs on the seat stays.
ETA: Loctite all fasteners with blue Loctite.